The present invention relates to a method for operating a synchronous motor comprising a first motor component and a second motor component, wherein the first and the second motor component are movable relative to each other. The invention further relates to an amplifier for operating such a synchronous motor as well as to a system comprising an amplifier and a synchronous motor.
Synchronous motors comprising a first and a second motor component which are configured in such a way that they may be moved relative to each other are known in various embodiments. In a rotary embodiment, one of the motor components may be rotated with regard to the other motor component. In what is referred to as a linear drive or linear motor, respectively, a first or a second motor component may be displaced with regard to each other in a linear movement (translational movement).
In all these various embodiments, the initiation of a movement is based on an interaction of magnetic fields. In this context, a magnetic field with rotary or translational movement (rotary field or, respectively, travelling field) is generated with the first motor component by means of electromagnets (current-carrying conductors or, respectively, coils). The second motor component comprises one or more permanent magnets so that a static magnetic field is provided with regard to the second motor component. The magnetic field of the first motor component interacts with the static field of the second motor component so that the first and second motor component are moved relatively to each other.
In order to generate a moving magnetic field, the electromagnets of the first motor component are actuated or, respectively, provided with a current at a timely displacement with regard to each other. This current transfer from one electromagnet to the next is also referred to as commutation. For effective operation of a synchronous motor, it is required to harmonize the point of time of the commutation with respect to the magnetic field of the second motor component.
In the case of a mechanical commutation, sliding contacts (usually in the shape of brushes) are used in a predetermined geometrical arrangement so that during a relative movement of the first and the second motor component the current flow is switched over accordingly. In this way, commutation may be realized involving relatively little complexity, however, wear on the brushes or, respectively, loss of the brushes during operation are disadvantageous, as well as undesired effects such as extensive sparking. Current synchronous motors are thus usually operated by means of electronic commutation. For this purpose, electronic control devices, also referred to as servo-amplifiers, are assigned to the motors in order to be able to generate the rotary field necessary for motor operation without wear and in a smooth manner.
Aligning the rotary magnetic field generated by means of the first motor component with the static magnetic field of the second motor component within the framework of an electronic commutation, however, requires knowledge of the spatial position of the first and the second motor component in relation to each other. This is significant at the start of the operation of a synchronous motor in order to allow for an effective operational mode with a maximum torque utilization. As a result, position-determining devices are used which are also referred to as encoders or, respectively, encoding systems. These may be configured as what is referred to as absolute encoders in order to allow for unambiguously determining the position of a motor component. It is, however, disadvantageous that the use of such an absolute encoder is attended by relatively high costs.
DE 10 2004 012 805 A1 describes a method for determining the angular position (orientation) of a rotor of an electric motor comprising permanent magnets. In the method, it is proposed to apply current pulses to windings (electromagnets) of a stator of the electric motor according to a predetermined pulse pattern, to measure the angular acceleration of the rotor occurring thereby by means of an acceleration sensor and to calculate the sought-after rotor position on the basis of these data.
EP 0 784 378 A2 refers to a method for determining the absolute rotor position of a synchronous machine. In this context, a magnetic field is generated by means of electromagnets of a stator, the rotary movement of a permanently energized rotor occurring thereby is detected and the orientation of the magnetic field of the stator is changed or, respectively, rotated within the framework of a control method until the rotary movement of the rotor comes to a halt. In this state, the orientations or, respectively, angular positions of the magnetic fields of rotor and stator are congruent, thus determining in the sought-after rotor position.
R. Schönfeld, W. Hofmann, “Elektrische Antriebe and Bewegungssteuerungen”, pp. 309-310, VDE (publisher), 2005 discloses methods for determining the position of initial angles of electric motors. According to one method, the application of a magnetic field is provided in order to align a permanently energized rotor therewith.
DE 44 07 390 A1 refers to a method for the start-up phase and the commutation of synchronous machines. This method proposes a test excitation of a rotor by applying polyphase-currents to windings of a stator. Moreover, the maximum torque achieved thereby is determined. The phase of the stator currents associated with the maximum torque is used as commutation angle for the initial operation of the synchronous motor.